Winding with two parallels per pole



Sept. 24, 1935. A. KILGORE WINDING WITH TWO PARALLELS PER POLE Filed April 12, 1934 2 Sheets-Sheet 1 Q 53' b5 5 6 b abbaab abbaafi 6 my my wfl WK A e e L WITNESSES ATTORNEY Sept. 24, 1935.

L. A. KILGOREI WINDING WITH TWO PARALLELS PER POLE] Filed April 12, 1954 2 Sheets-Sheet 2 B; 5 A3 b a, a 6, a b, 4 a, b 4 0 k P026 "7.

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H 673 b3 b, a2 b3 b3 67 0 b2 5 a 62, b b 6 52 3 A, B/ 2 A 5 .B/ A3 53 B, A A 53 53 A A B2 B A A B INVENTOR Lee A. KZ/gore.

ATTORNEY Patented Sept. 24, 1935 UNITED STATES PATENT OFFICE WINDING WITH TWO. PARALLELS PER POLE vania Application April 12, 1934, Serial No. 720,220

15 Claims.

My invention relates to alternating-current windings for dynamo-electric machines, and while it is not limited to any particular type of alternating-current machine, it will probably find its broadest application in turbo-generators which are of relatively low voltage, that is, a voltage which is unusually low, considering the size and kilovolt-ampere rating of the machine.

Heretofore, when it has been necessary to design such a machine for unusually low voltages, it has been necessary to resort to delta connection of the winding, which is less desirable than the usual star connection, and in extreme cases it has been necessary to resort to a design in which only one conductor is placed in each slot, in order to obtain the requisite low terminal voltage.

The one-conductor-per-slot winding is very objectionable from the design standpoint and is to be avoided whenever possible, for various reasons, including the additional expense of build-' ing the machine because of the special bending of the end-connections of the winding and the excessive load loss which is usually incumbent. By load loss, I means the extra losses due to load current other than the normal copper loss. Most of this extra loss is in the rotor member, some being, however, in the stator teeth, due to the special flux distribution obtained with the single-conductor-per-slot winding. These excessive losses mean lower efficiency, and in some cases a bigger machine.

All low-voltage machines, that is, machines having a voltage which is low for their size, have heretofore been designed to have as many circuits in parallel as there are pairs of poles, or double this number if the double capacity is needed, in order to avoid the high voltage which would be obtained with the series connection of the circuits. Heretofore, however, no machine to my knowledge has been built having two parallel circuits per pole as distinguished from two parallel circuits per pair of poles. The reason for this is possibly to be found in the apparent impossibility of causing the respective voltages of the paralleled circuits to be exactly equal in both magnitude and phase, within the limits of a single pole. It has been, and still is, necessary to go through two electrical poles of the winding before it is possible to come out with two parallel circuits having voltages which are exactly matched in both phase and magnitude.

According to my invention, I make a special slot-arrangement of the paralleled windings in each pole, so that I can parallel them under each pole.

By means of my invention, utilizing two parallels per pole, I obtain just half the voltage of the winding-arrangement utilizing two parallels per pair of poles.

It is true that I cannot exactly balance both 5 the magnitude and the phase of the voltages of the paralleled circuits, and hence I obtain circulating currents in the two paralleled circuits. These circulating currents increase the losses in one of the circuits and decrease the losses in 10 the other, the total efiect being more losses than are obtained with two parallels per pair of poles, but I obtain a machine with only half the voltage that it would have if the windings were paralleled per pair of poles rather than per pole. 15

Moreover, I find that the heating effects in the paralleled winding-circuit which contains the additional losses are intermediate between the total average percentage losses of the entire machine and the percentage losses in the particular 20 circuit in question. This is because of the proximity of the other circuit, in which the losses are less than the normal load-current-responsive losses, due to the fact that the circulating current is bucking against the load current in this cir- Z5 cuit. The conductivity of heat within the machine, from one winding-circuit to the other, is sufficiently good to give considerable gain as a result of this averaging effect. Thus it is possible to accommodate considerable circulating 30 current without excessive temperature.

Having established the fact, therefore, that the heating effects in my machine will be reasonable, and assuming that it is necessary to provide a machine having half the voltage that would be obtained in the ordinary design utilizing starconnected windings with two parallels per pair of poles, it remains to determine how my design compares in efficiency with other means for getting the same low voltage. To change the wind- 40 ing from star to delta connection would reduce the voltage to only .577 times it former value, and would have the disadvantage of always having some circulating current in the delta, such circulating current being quite excessive in some 45 machines, and it would require special equipment to provide a neutral connection which is required in many instances. If resort were had to a one-conductor-per-slot design, in order to obtain half of the voltage of the normal design, the 50 losses would be higher than in my design and the winding would be much more expensive to build. Furthermore, by my design, I am enabled to get down to half the voltage which was previously possible in extreme cases, when both the all) delta connection and the one-conductor-per-slot design are resorted to, so that I can attain voltages which are lower than were previously possible, regardless of the efiiciency.

An important feature of my design is that I find it possible, by simple expedients, to very greatly reduce the magnitude of the circulating currents which are obtained by reason of the potentiahdifierence between the paralleled cir-- cuits. These circulating currents are limited by the impedance, which is almost entirely re acta-nce, of the closed parallel circuit. Reactances are usually expressed, by the design engineer, in terms of the percentage of the rated (terminal) voltage of the machine which is obtained when rated (full-load) current is passed through the reactance in question. When the two parallel circuits are connected in series rather than in parallel, the equivalent reactance is quadrupled, so that the effective reactance against circulating currents is four times the load-current reactance, assuming that the reactance to load-current is the same as the reactance to circulating current in each of the individual circuits. I have found, however, that the effects of mutual reactance between the two circuits is to increase the circulating-current reactance, for some values of chording of the winding, and to decrease it for other windingpitches, depending upon the manner in which coil-sides of different circuits are thrown into the same slot. Naturally, pitches giving a decreased circulating-current reactance are to be avoided, because this means more circulating current and more losses. It is also obviously possible to utilize small auxiliary reactances in series with the paralleled winding circuits, in order to supplement the circulating-current reactance where necessary, and the size and cost of such auxiliary reactances would not be exorbitant I have found it possible and preferable, however, to obtain all the circulating-current reactance necessary, by means of proper attention to the design, particularly to the pitch of the winding, or chording.

In accordance with the design-principles herein-above outlined, therefore, I have found that it is not only practicable to utilize parallelings between the winding-groups under each pole, even though the voltages are not exactly matched, but I have found that it is preferable to do so, in lieu of other expedients which have heretofore been resorted to to obtain low terminal voltages, such as the delta-winding connection and the one-conductor-per-slot design, and I have for the first time made it possible, by any means, to obtain a practical machine having a terminal voltage which is just half of the ultimate minimum of terminal voltage which was heretofore obtainable in any given size of machine.

My invention is illustrated by way of example in the accompanying drawings, wherein:

Figure 1 is a diagrammatic view showing a development of the primary member, with the wiring for one phase of a three-phase, two-pole, 48-slot generator with four parallel winding-circults, or two parallels per pole;

Fig. 2 is a sectional view of the developed primary winding of the same machine;

Figs. 3, 4 and 5 are diagrammatic sectional views of the developed primary members of machines embodying modifications; and

Figs. 6 and 7 are vector diagrams which will be referred to in the description.

In Fig. l, I have shown my invention as applied to a two-pole, four-circuit generator having a field member which is diagrammatically indicated by north and south poles, N and S, and a primary or armature member consisting of a magnetizable'core 3 having 48 slots 4, and a two-layer Winding consisting of conductors or coil-sides 5 and end-connections 6. Only one phase as shown in Fig. 1, for convenience of illustration, the coil-sides which lie in the bottoms of the slots being indicated by shorter lines than the coil sides which lie in the tops of the slots, The conductors for all three phases are shown in Fig. 2.

There are two circuits for each pole of the winding, that is, for each electrical degrees. The coil sides of the two circuits under pole No. l in the particular instance shown in the drawings are indicated by the letters a and b, respectively, and the coil-sides for the two circuits under pole No. 2 are indicated by the letters A and B, respectively. In Fig. 2, subscripts l, 2, and 3 are added to indicate the three different phases.

It will be noted that each of the coil-sides 5,

whether of circuit-a or circuit-b, is in effect a single conductor, which means that it may be either a single solid conductor or it may be laminated into a plurality of strands which may be transposed in position, in accordance with a wellknown method, in order to reduce eddy-currents and to bring about a more effective utilization of the total cross-sectional area of the entire conductor. It will be understood that the strands of each conductor, if stranding is utilized, are all connected together at their ends, so that, in effect, s

they collectively constitute but a single conductor or coil-side.

It will be noted that each coil has two coilsides, and since the winding is a double-layer winding, that is, with one coil-side of each coil lying in the top of the slot and the other coilside of the coil lying in the bottom of another slot, there are obviously as many coils as slots, or 48, in the machine shown in Fig. 1. Since there are two poles and three phases, this leaves 8 coils per phase per pole.

According to my invention, there are two circuits per phase per pole, and each of these circuits consists of four coils connected in series. The coil-sides per phase per pole fall into two groups of eight coilsides each, the conductors a and b of the several groups falling in the sequence abbaabba in each case. The general idea is to choose the coil-sides, which lie in diflerent positions with respect to the center-line of the group, so as to produce a minimum difference in voltage between the two circuits under each pole.

Reference to Fig. 6 will give a. better idea of how the voltages of the parallel circuits may be calculated. This figure shows a simpler case in which there are only four slots per phase per pole, instead of eight, the conductors being arranged in the order abba. In a three-phase, two-pole machine, each phase belt will be 60" wide and the conductors will be disposed symmetrically with I respect to the axis 1 of the phase belt, or the center-line of the group of coil-sides. The individual voltages es, en, eb, '30., induced in the four conductors abba will be equal in magnitude but displaced in phase, the phase-displacement, in a two-pole machine, being commensurate with the spacial displacement, giving the four voltages ea, eb', 6b, ea, (Fig. 6). Since all the a-conductors are connected in series, their voltages add, to give the circuit-a resultant voltage Ea. In like manner, the small voltages ea add, to give the resultant circuit-b voltage Eb. The voltage-difference between the two circuits is (Es-Eb). It is noted, in Fig. 6, that these resultant voltages Ea and Eb are in phase coincidence with each other but slightly different in magnitude.

In Fig. '7, I have shown the method of calculating the circuit-voltages in a design in which there are six slots per phase per pole, in which the optimum arrangement of the coil-sides is in the order abbactb. The same vector designations are utilized as in Fig. 6, so that it is easy to see how the resultant voltages Ea and Eb are obtained, each voltage, in this case, being the resultant of three coil-side voltages, as indicated. It will be noted, in this case, that the resultant voltages E8. and El) are equal in magnitude, but slightly displaced in phase, so that there is a slight voltagediiierence (EaEb) between them.

In Fig. 3 I have shown, by way of example, one phase of one pole of a machine having six slots per phase per pole, in accordance with the vector illustration in Fig. 7.

The windings shown in Figs. 1, 2, and 3 have been double-layer windings.

In Fig. 4 I illustrate my invention as being applied to a single-layer winding having but one conductor or coil-side in each slot. Here, there are only one-half as many coils as there are slots, because each coil-side occupies a whole slot rather than half of a slot. Fig. 4 illustrates a 48-slot, three-phase, two-pole machine with a single conductor-per-slot winding which would consist, therefore, of only 24 coils. Only one phase and one pole is illustrated in Fig. 4. Although there are eight slots per phase per pole, the same as in Figs. 1 and 2, the fact that there are only four coils per phase per pole makes this machine equivalent, in its vector diagram, to a double-layer winding in which there are four slots (or coils) per phase per pole, so that Fig. 6 shows the vector relations of the voltages induced in the. coils of the respective paralleled circuits a1 and bi in Fig. 4.

There is a third type of winding to which my invention applies, in addition to the double-layer and single-layer windings previously described, and that is what is known as a fractional-slot winding, which is a winding in which the number of slots per pole comes out a Whole number and a fraction, so that it is necessary to go through two or more poles before any balanced group of coils is obtained. In that case, the balanced group of coils must be considered as the unit to which my invention is applied rather than the windinggroups comprising one phase under each pole, as in the previous cases.

Throughout this specification and in the appended claims I utilize the term phase-belt to designate the units to which my invention applies, the same consisting of one phase per pole in each case, except in the case of a fractional-slot winding, when it consists of a balanced group including as many poles as may be necessary to make the number of slots in the group a whole number without any fraction.

Fig. 5 illustrates my invention as applied to a fractional-slot winding, in which the number of slots per phase per pole is 1%, so that the balanced group of coils embraces five poles and consists of a whole number (eight) which is a multiple of the number of parallel circuits (two) into which the group is to be divided. In this case there are two parallel circuits of four coils each, in each balanced group of eight coils. Noting that the corresponding slots under each successive pole are displaced 7 electrical degrees from those of the preceding pole, it will be seen that there are three groups or phases, each having eight different conductor-positions, or phase-positions, separated in phase by l /2 1. some multiple of 180, as seen, for example, in the conductors H, i2, l3, l4, l5, iii-ll, l8. These conductors show the same sequence as the winding in Fig. 2, namely abbaabba, showing the equivalence of the two phase-belts in Figs. 2 and 5, respectively.

The effect of chording must be considered, in its relation to the reactance of the winding, particularly the reactance to the circulating currents. It is desirable that this reactance shall be as large as conveniently possible, as it reduces the magnitude of the circulating currents, and reduces the extra losses which are entailed by the subdivision of the winding into parallel circuits in each phase-belt, instead of parallel circuits for each pair of poles or phase-belts. I have also considered the slot reactances and voltage differences as applied to various harmonics which may appear in the voltage-wave, and find that the heating effects of the circulating currents due to these harmonics are rather negligible.

Confining our attention, therefore, to the fun-- damental voltage wave, the calculated efiect of chording is indicated in. Tables I, II and III for machines having eight, six and four slots (or conductor-positions) per phase per pole, respectively, and having different degrees of chording. The effect of chording on the circulating-current reactance is best indicated or measured by what I term a reduction-factor ratio K/k, where k is the ratio of the slot reactance of a chorded winding to that of a full-pitch winding, for normal load-currents, and K is the ratio of the slot I reactance of a chorded winding to that of a fullpitch winding, for the circulating current caused by the difference in voltage between paralleled phase-belts.

This ratio is applicable only to double-layer windings, as chording does not affect the reac tance of one-conductor-per-slot windings. because of the fact that each coil side lies in a separate slot. Fortunately, the slot reactance is inherently high in oneconductor-per-slot windings, so that the problem of circulating currents, for any given potential difierence between the paralleled circuits, is not as serious as in some double-layer windings.

The reduction-factor ratios K/Ic for various double-layer windings are indicated in the following tables:

TABLE I K/k for 8 slots (01' conductor-positions) per phase per pole (01' phase-belt) Coil pitch 10/24 11/24 1/2 13/24 14/24 15/24 2/3 17/24 18/24 1924 56 2124 22 24 23 24 10 K/k 1.6 1.0 1.0 1.0 .60 .82 1.0 .72 .23 55 1(0 (56 (07 (48 1:0

TABLE II K/lt for 6 slots (or conductor-positions) per phase per pole (or phase-belt) (loll pitch 8/18 1/2 10/18 11/18 2/3 13/18 14/18 5/0 16/18 17/18 1.0 K/k 2.0 1.0 .80 .75 1.0 .48 .60 .72 .27 .30 1.0

TABLE III K/k for 4 slots (or c0nduct01'-p0sit'0ns) per phase per pole (or phasabelt) Coil pitch 5/12 1/2 7/12 2/3 0/12 5/0 11/12 1.0 K/k 1.0 .0 1.0 .70 .15 .33 1.0

Any coil pitch, or chording, which gives a reduction-factor ratio K/k of .6 or more, will in general be suitable for use with my invention, although better results will be obtained, so far as reduction of circulating currents is concerned, with chordings which give the larger ratios.

In Figs. 1 and 2, for example, I have shown a machine having eight slots per phase per pole, with a two-third pitch winding, giving a reduction-factor ratio of K/Ic equal to 1.0, as shown in Table I. In Fig. 5 I have shown a machine having eight conductor-positions per phase-belt, chorded to 10/24 pitch, giving a reduction-factor ratio of K/k equal to 1.6, as shown in the same table. In Fig. 3 I have shown a winding having six coils per phase per pole, with a chording of 8/18, which, cording to Table II, gives a reduction-factor ratio of K/k equal to 2.0. The windings in Figs. 3 and 5 have the optimum values of chording, from the standpoint of reduction of circulating currents.

While I have illustrated my invention in several forms of embodiment, it will be obvious that it is by no means limited to the illustrated examples, as I have merely undertaken to set forth the general principles. I desire, therefore, that the appended claims shall be accorded the broadest construction consistent with their language and the prior art.

I claim as my invention:

1. A dynamo-electric machine having an alternating-current winding and a multi-slot core therefor, said winding having a plurality of phase-belts, each phase-belt comprising a plurality of parallel-connected circuits in different slots, each circuit comprising a plurality of coils connected in series, each coil having two coil-sides, each of the coil-sides of any given circuit lying in a. different slot, the number of coil-sides in each of said parallel-connected circuits being the same, and the disposition of the coil-sides with respect to the central axis of each phase-belt being such as to produce substantially the minimum possible voltage difference between the resultant generated voltages in the circuits connected together in parallel.

2. A dynamo-electric machine having an alterhating-current winding and a multi-slot core therefor, said winding having a plurality of phase-belts, each phase-belt comprising a plurality of parallel-connected circuits in different slots, each circuit comprising a plurality of coils connected in series, each coil having two coil-sides, each of the coil-sides of any given circuit lying in a. diflerent slot, the number of coil-sides in each of said parallel-connected circuits being the same, the disposition of the coil-sides with respect to the central axis of each phase-belt being such as to produce substantially the minimum. possible voltage difference between the resultant generated voltages in the circuits connected together in parallel, and the winding having such chording as to give a high reactance to the flow of circuthe disposition of the coil-sides with respect to the central axis of each phase-belt being such. as to produce substantially the minimum possible voltage difference between the resultant generated voltages in the circuits connected together in parallel, and the winding having such chording as to give a reduction-factor ratio K/k of at least .6, where k is the ratio of the slot reactance of a chorded winding to that of a full-pitch winding, for normal load-currents, and K is the ratio of the slot reactance of a chorded winding to that of a full-pitch winding, for the circulating current caused by said difference in voltage.

A dynamo-electric machine having an alternating-current double-layer winding and a multislot core therefor, said winding having a plurality of parallel-connected circuits per phase per pole, in difierent slots, each circuit comprising a plurality of coils connected in series, each coil having two coil-sides, each of the coi1-sides of any given circuit lying in a different slot, the number of coil-sides in each of said parallel-connected circuits being the same, and. the disposition of the coil-sides with respect to the central axis of each phase-belt being such as to produce substantially the minimum possible voltage-diiference between slot core therefor, said winding having a plurality of parallel-connected circuits per phase per pole, in different slots, each circuit comprising plurality of coils connected in series, each coil having two coil-sides, each of the coil-sides of any given circuit lying in a different slot, the number of coil-sides in each of said parallel-connected circuits being the same, the disposition of the coil-sides with respect to the central axis of each phase-belt being such as to produce substantially the minimum possible voltage-difference between the resultant generated voltages in the circuits connected together in parallel, and the winding having such chording as to give a high reactance to the flow of circulating current caused by said difference in voltage.

6. A dynamo-electric machine having an alternating-current double-layer winding and a multi-slot core therefor, said winding having a plurality of parallel-connected circuits per phase per pole, in diiferent slots, each circuit comprising a plurality of coils connected in series, each coil having two coil-sides, each of the coilsides of any given circuit lying in a different slot, the number of coil-sides in each of said parallel-connected circuits being the same, the disposition of the coil-sides with respect to the central axis of each phase-belt being such as to produce substantially the minimum possible voltage-difierence between the resultant generated voltages in the circuits connected together in parallel, and the winding having such chording as to give a reduction-factor ratio K/k of at least .6, where k is the ratio of the slot'reactance of a chorded winding to that of a fullpitch winding, for normal load-currents, and K is the ratio of the slot reactance of a chorded winding to that of a full-pitch winding, for the circulating current caused by said difference in voltage. I

'7. A dynamo-electric machine having a fractional-slot alternating-current winding characterized by having a phase-belt comprising a. plurality of parallel-connected circuits in different slots, each circuit comprising a plurality of coils connected in series, each coil having two coilsides, each of the coil-sides of any given circuit lying in a different slot, the number of coil-sides in each of said parallel-connected circuits being the same, and the disposition of the coil-sides with respect to the central of each phasebelt being such as to produce substantially the minimum possible voltage-difference between the resultant generated voltages in the circuits connected together in parallel.

8. A dynamo-electric machine having a fractional-slot alternating-current winding characterized by having a phase-belt comprising a plurality of paralle1connected circuits in different slots, each circuit comprising a plurality of coils connected in series, each coil having twocoilsides, each of the coil-sides of any given circuit lying in a different slot, the number of coilsides in each of said parallel-connected circuits being the same, the disposition of the coil-sides with respect to the central axis of each phasebelt being such as to produce substantially the minimum possible voltage-diiference between the resultant generated voltages in the circuits connected together in parallel, and the winding having such chording as to give a high reactance to the flow of circulating current caused by said difference in voltage.

9. A dynamo-electric machine having a fractional-slot alternating-current winding characterized by having a phase-belt comprising a plurality of parallel-connected circuits in different slots, each circuit comprising a plurality of coils- 'ratio of the slot reactance of a chorded winding to that of a full-pitch winding, for normal load-currents, and K is the ratio of the slot reactance of a chorded winding to that of a full-pitch winding, for the circulating current caused by said difference in voltage.

10. A dynamo-electric machine having an alternating-current one-conductor-per-slot winding and a multi-slot core therefor, said winding having a plurality of parallel-connected circuits per phase per pole, in different slots, each circuit comprising .a plurality of coils connected in .series, each coil having two coil-sides, the number of coil-sides each of said parallel-connected circuits being the same, and the disposition of the coil-sides with respect to the central axis of each phase-belt being such as to produce substantially the minimum possible voltage-difierence between the resultant generated voltages in the circuits connected together in parallel.

11. A dynamo-electric machine having an alternating-current winding and a multi-slot core therefor, said winding having a plurality of phase-belts, each phase-belt comprising a plurality of circuits in different slots, each circuit comprising a plurality of coils connected in series, each coil having two coil-sides, each of the coil sides of any given circuit lying in a differ ent slot, the number of coil-sides in each of said circuits being the same, and the disposition of the coil-sides with respect to the central axis of said phase-belt being such as to produce substantially the minimum possible voltage-difference between the resultant generated voltages in the circuits of any given phase-belt.

12. A dynamo-electric machine having an alternating-current winding and a multi-slot core therefor, said winding having a plurality of phasebelts, each phase-belt comprising a plurality of circuits in different slots, each circuit comprising a plurality of coils connected in series, each coil having two coil-sides, each of the coil-sides of any given circuit lying in a different slot, the number of coil-sides in each of said circuits being the same, the disposition of the coil-sides with respect to the central axis of each phasebelt being such as to produce substantially the minimum possible voltage-difference between the resultant generated voltages in the circuits of any given phase-belt, and the winding having such chording as to give a high reactance to the flow of such circulating current as would be caused by said difference in voltage.

13. A dynamo-electric machine having an alternating-current winding and a multi-slot core therefor, said winding having a plurality of phasebelts, each phase-belt comprising a plurality of circuits in defferent slots, each circuit comprising a plurality of coils connected in series, each coil having two coil-sides, each of the coil-sides of any given circuit lying in a different slot, the number of coil-sides in each of said circuits being the same, the disposition of the coil-sides with respect to the central axis of each phasebelt being such as to produce substantially the minimum possible voltage-difference between the resultant generated voltages in the circuits of any given phase-belt, and the winding having such chording asto give a reduction-factor ratio K/k of at least .6, where k is the ratio of the slot reactance of a chorded winding to that of a full-pitch winding, for normal load currents, and K is the ratio of the slot reactance of a chorded winding to that of a full-pitch winding, for the circulating current that would be caused by said diiference in voltage.

14. A dynamo-electric machine having an alternating-curren-t winding and a multi-slot core therefor, said winding having two parallel-connected circuits per phase per pole in different slots, each circuit comprising a plurality of coils connected in series, each coil having two coil sides, each of the coil-sides of any given circuit lying in a different slot, the number of coil-sides in each of said parallel-connected circuits being the same, and the disposition of the coil-sides being in the order abbaabba, where the letter a indicates the coil-sides of one circuit, and the letter b indicates the coil-sides of the other circuit.

15. A dynamo-electric machine having an alternating-current winding and a multi-slot core therefor, said winding having two circuits per phase per pole, in different slots, each circuit comprising a plurality of coils connected in series, each coil having two coil-sides, each of the coilsides of any given circuit lying in a diiferent slot, the number of coil-sides in each of said circuits being the same, and the disposition of the coil sides being in the order abbaabba, Where the letter (1 indicates the coil-sides of one circuit, and

the letter 19 indicates the coil-sides of the other 10 

